1. Field of the Invention
This invention relates to luminance signal/chrominance signal separating circuits and noise reduction circuits, and more particularly, to a luminance signal/chrominance signal separating circuit and a noise reduction circuit using a 3 line logical comb filter in a magnetic recording reproducing device such as a video circuit recorder (hereinafter referred to as a VCR).
2. Description of the Background Art
A VCR has been proposed using a comb filter, operating as a Y/C separating filter during recording for separating an input video signal into a luminance signal Y and a chrominance signal C (Y/C separation), and operating as a noise reduction circuit during reproduction for removing the small noise components in the reproduced luminance signal.
As an example of a comb filter, the so-called 3 line logical comb filter is known, such as the one described in U.S. Pat. No. 4,524,382, in which the comb type chrominance signal passing characteristic is implemented by using the bandpass filter (BPF) of each line for separating the chrominance signal components of 3 lines from the video signals of 3 lines to be logically processed.
A color TV signal (hereinafter referred to as video signal) of the NTSC system has the spectrum of luminance signal components and the spectrum of chrominance signal components alternately interleaved as shown schematically in the frequency spectrum diagram of FIG. 1 to avoid beat disturbance on the screen. Generally, when there is no difference between adjacent upper and lower horizontal lines, that is, when there is line correlation, luminance signal components are concentrated in the vicinity of the spectrum of n times (n is integer) the horizontal frequency f.sub.H (solid lines Y.sub.1), while chrominance signal components are concentrated in the vicinity of the spectrum of (n+1/2) f.sub.H around 3.58 MHz (broken lines C). That is to say, Y signal components and C signal components are mutually frequency interleaved by (1/2) f.sub.H. On the contrary, when there is difference between adjacent upper and lower horizontal lines, that is, when there is no line correlation, non-correlation components Y.sub.2 and Y.sub.3 of the Y signal are also generated in a low frequency spectrum region other than in the vicinity of Y.sub.1 which is the frequency multiplied output of the horizontal frequency, i.e., in the frequency spectrum region shown by the broken lines (FIG. 1).
For example, in a VCR recording mode, a filter having a comb type chrominance signal band-pass characteristic periodically repeated at regular intervals in accordance with the repetition of the spectrum shown in FIG. 1 is required to extract chrominance signal components from the input video signal having such a frequency spectrum.
The previously mentioned 3 line logical comb filter is used as the filter having such a comb type band-pass characteristic of a chrominance signal. However, since the frequency characteristic of the BPF separating the chrominance signal components by each horizontal line from the video signals of three horizontal lines in the 3 line logical comb filter is relatively gentle, each BPF will separate the signal components of a wide band including the components of the luminance color band. This causes the chrominance signal components provided as the output of the 3 line logical comb filter by logical processing in accordance with the output of the BPFs to include the non-correlation components Y.sub.2 and Y.sub.3 of the low frequency range and the high frequency range of the luminance signal. In the case where luminance signal components are extracted by subtracting such chrominance signal components from the input video signal, even the non-correlation components are removed from the luminance signal components. This eliminates the luminance signal variation between the upper and lower lines, that is, in the vertical direction, resulting in a problem of degradation in non-correlation luminance signals, i.e., in oblique resolution of the picture on the screen.
The 3 line logical comb filter used as a Y/C separating filter in a recording mode as described above may be used as a noise reduction circuit that removes the noise components in the luminance signal during the reproduction mode of the VCR. However such a noise reduction circuit had a similar problem of the non-correlation components of the reproduced luminance signal being removed due to the reason described above with respect to the operation during recording.